The present invention generally relates to air-conditioning systems, and more particularly to automotive air-conditioning systems configured to disengage the compressor of the system when the vehicle is under relatively high loads.
The efficiency of an automotive engine's operation can be increased by engaging the compressor of its air-conditioning system to operate the system only when the engine is not loaded or only lightly loaded. As a result, various methods have been proposed for interrupting the operation of air-conditioning systems for the purpose of reducing the load on a vehicle under certain conditions, for example, during acceleration and when operating on an incline. As an example, certain systems have been proposed that engage and disengage a clutch through which power is delivered to the compressor based on one or more parameters that are indicative of the load on the engine.
Engine intake manifold vacuum is a good indicator of engine loading, and therefore air-conditioning systems have been proposed that utilize the engine intake manifold or plenum vacuum level as a parameter for controlling the operation of the compressor. Engine intake manifold vacuum and engine loading are inversely proportional, such that a high manifold vacuum level (in other words, a low manifold absolute pressure relative to ambient atmospheric pressure) corresponds to a low engine load and lower manifold vacuum levels (in other words, manifold absolute pressures relatively closer to ambient atmospheric pressure) correspond to higher engine loads. Therefore, the compressor clutch is disengaged if the manifold pressure level rises above a predetermined threshold indicative of a high engine load, for example, during acceleration, and allowed to re-engage once the manifold pressure level has dropped below a predetermined threshold, for example, after the desired vehicle speed is attained and during vehicle coasting. In this manner, the system operates to override the heating, ventilating, and air-conditioning (HVAC) computer of a vehicle and its control of the compressor.
A complicating factor in the incorporation of systems of the type described above occurs if a vehicle is operated for prolonged periods at high engine loads, for example, when a long incline is encountered. Aside from the annoyance of the passenger compartment becoming warmer than desired, safety issues can arise if weather conditions require that the vehicle's HVAC controls are set to defrost. As a possible remedy, U.S. Pat. No. 5,228,305 proposed means by which the threshold at which an air-conditioning compressor is disengaged can be manually adjusted by the vehicle's operator. However, the necessity for the operator to know how to make such manual adjustments can be burdensome and potentially dangerous.